diff --git a/src/fix_addforce.cpp b/src/fix_addforce.cpp
index 48b006eb2d..b4d13ff5fa 100644
--- a/src/fix_addforce.cpp
+++ b/src/fix_addforce.cpp
@@ -38,7 +38,7 @@ FixAddForce::FixAddForce(LAMMPS *lmp, int narg, char **arg) :
     Fix(lmp, narg, arg), xstr(nullptr), ystr(nullptr), zstr(nullptr), estr(nullptr),
     idregion(nullptr), region(nullptr), sforce(nullptr)
 {
-  if (narg < 6) error->all(FLERR, "Illegal fix addforce command");
+  if (narg < 6) utils::missing_cmd_args(FLERR, "fix addforce", error);
 
   dynamic_group_allow = 1;
   scalar_flag = 1;
@@ -77,25 +77,25 @@ FixAddForce::FixAddForce(LAMMPS *lmp, int narg, char **arg) :
   int iarg = 6;
   while (iarg < narg) {
     if (strcmp(arg[iarg], "every") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal fix addforce command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "fix addforce every", error);
       nevery = utils::inumeric(FLERR, arg[iarg + 1], false, lmp);
-      if (nevery <= 0) error->all(FLERR, "Illegal fix addforce command");
+      if (nevery <= 0) error->all(FLERR, "Invalid fix addforce every argument: {}", nevery);
       iarg += 2;
     } else if (strcmp(arg[iarg], "region") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal fix addforce command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "fix addforce region", error);
       region = domain->get_region_by_id(arg[iarg + 1]);
       if (!region) error->all(FLERR, "Region {} for fix addforce does not exist", arg[iarg + 1]);
       idregion = utils::strdup(arg[iarg + 1]);
       iarg += 2;
     } else if (strcmp(arg[iarg], "energy") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal fix addforce command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "fix addforce energy", error);
       if (utils::strmatch(arg[iarg + 1], "^v_")) {
         estr = utils::strdup(arg[iarg + 1] + 2);
       } else
-        error->all(FLERR, "Illegal fix addforce command");
+        error->all(FLERR, "Invalid fix addforce energy argument: {}", arg[iarg + 1]);
       iarg += 2;
     } else
-      error->all(FLERR, "Illegal fix addforce command");
+      error->all(FLERR, "Unknown fix addforce keyword: {}", arg[iarg]);
   }
 
   force_flag = 0;
diff --git a/src/fix_indent.cpp b/src/fix_indent.cpp
index 766ec88d8e..f38ab5efa6 100644
--- a/src/fix_indent.cpp
+++ b/src/fix_indent.cpp
@@ -83,7 +83,7 @@ FixIndent::FixIndent(LAMMPS *lmp, int narg, char **arg) :
     if (cdim == 0 && !pstr) pvalue *= xscale;
     else if (cdim == 1 && !pstr) pvalue *= yscale;
     else if (cdim == 2 && !pstr) pvalue *= zscale;
-  } else error->all(FLERR,"Illegal fix indent command");
+  } else error->all(FLERR,"Unknown fix indent keyword: {}", istyle);
 
   varflag = 0;
   if (xstr || ystr || zstr || rstr || pstr) varflag = 1;
@@ -121,37 +121,37 @@ void FixIndent::init()
   if (xstr) {
     xvar = input->variable->find(xstr);
     if (xvar < 0)
-      error->all(FLERR,"Variable name for fix indent does not exist");
+      error->all(FLERR,"Variable {} for fix indent does not exist", xstr);
     if (!input->variable->equalstyle(xvar))
-      error->all(FLERR,"Variable for fix indent is invalid style");
+      error->all(FLERR,"Variable {} for fix indent is invalid style", xstr);
   }
   if (ystr) {
     yvar = input->variable->find(ystr);
     if (yvar < 0)
-      error->all(FLERR,"Variable name for fix indent does not exist");
+      error->all(FLERR,"Variable {} for fix indent does not exist", ystr);
     if (!input->variable->equalstyle(yvar))
-      error->all(FLERR,"Variable for fix indent is not equal style");
+      error->all(FLERR,"Variable {} for fix indent is invalid style", ystr);
   }
   if (zstr) {
     zvar = input->variable->find(zstr);
     if (zvar < 0)
-      error->all(FLERR,"Variable name for fix indent does not exist");
+      error->all(FLERR,"Variable {} for fix indent does not exist", zstr);
     if (!input->variable->equalstyle(zvar))
-      error->all(FLERR,"Variable for fix indent is not equal style");
+      error->all(FLERR,"Variable {} for fix indent is invalid style", zstr);
   }
   if (rstr) {
     rvar = input->variable->find(rstr);
     if (rvar < 0)
-      error->all(FLERR,"Variable name for fix indent does not exist");
+      error->all(FLERR,"Variable {} for fix indent does not exist", rstr);
     if (!input->variable->equalstyle(rvar))
-      error->all(FLERR,"Variable for fix indent is not equal style");
+      error->all(FLERR,"Variable {} for fix indent is invalid style", rstr);
   }
   if (pstr) {
     pvar = input->variable->find(pstr);
     if (pvar < 0)
-      error->all(FLERR,"Variable name for fix indent does not exist");
+      error->all(FLERR,"Variable {} for fix indent does not exist", pstr);
     if (!input->variable->equalstyle(pvar))
-      error->all(FLERR,"Variable for fix indent is not equal style");
+      error->all(FLERR,"Variable {} for fix indent is invalid style", pstr);
   }
 
   if (utils::strmatch(update->integrate_style,"^respa")) {
diff --git a/src/fix_setforce.cpp b/src/fix_setforce.cpp
index 6a88e3ab17..8bd5477f76 100644
--- a/src/fix_setforce.cpp
+++ b/src/fix_setforce.cpp
@@ -37,7 +37,7 @@ FixSetForce::FixSetForce(LAMMPS *lmp, int narg, char **arg) :
     Fix(lmp, narg, arg), xstr(nullptr), ystr(nullptr), zstr(nullptr), idregion(nullptr),
     region(nullptr), sforce(nullptr)
 {
-  if (narg < 6) error->all(FLERR, "Illegal fix setforce command");
+  if (narg < 6) utils::missing_cmd_args(FLERR, "fix setforce", error);
 
   dynamic_group_allow = 1;
   vector_flag = 1;
@@ -77,13 +77,13 @@ FixSetForce::FixSetForce(LAMMPS *lmp, int narg, char **arg) :
   int iarg = 6;
   while (iarg < narg) {
     if (strcmp(arg[iarg], "region") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal fix setforce command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "fix setforce region", error);
       region = domain->get_region_by_id(arg[iarg + 1]);
       if (!region) error->all(FLERR, "Region {} for fix setforce does not exist", arg[iarg + 1]);
       idregion = utils::strdup(arg[iarg + 1]);
       iarg += 2;
     } else
-      error->all(FLERR, "Illegal fix setforce command");
+      error->all(FLERR, "Unknown fix setforce keyword: {}", arg[iarg]);
   }
 
   force_flag = 0;
diff --git a/src/fix_temp_rescale.cpp b/src/fix_temp_rescale.cpp
index 41fb49e3f4..f4f462c968 100644
--- a/src/fix_temp_rescale.cpp
+++ b/src/fix_temp_rescale.cpp
@@ -40,10 +40,10 @@ FixTempRescale::FixTempRescale(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
   tstr(nullptr), id_temp(nullptr), tflag(0)
 {
-  if (narg < 8) error->all(FLERR,"Illegal fix temp/rescale command");
+  if (narg < 8) utils::missing_cmd_args(FLERR, "fix temp/rescale", error);
 
   nevery = utils::inumeric(FLERR,arg[3],false,lmp);
-  if (nevery <= 0) error->all(FLERR,"Illegal fix temp/rescale command");
+  if (nevery <= 0) error->all(FLERR, "Invalid fix temp/rescale argument: {}", nevery);
 
   restart_global = 1;
   scalar_flag = 1;
@@ -106,9 +106,9 @@ void FixTempRescale::init()
   if (tstr) {
     tvar = input->variable->find(tstr);
     if (tvar < 0)
-      error->all(FLERR,"Variable name for fix temp/rescale does not exist");
+      error->all(FLERR,"Variable {} for fix temp/rescale does not exist", tstr);
     if (input->variable->equalstyle(tvar)) tstyle = EQUAL;
-    else error->all(FLERR,"Variable for fix temp/rescale is invalid style");
+    else error->all(FLERR,"Variable {} for fix temp/rescale is invalid style", tstr);
   }
 
   int icompute = modify->find_compute(id_temp);
@@ -195,7 +195,7 @@ void FixTempRescale::end_of_step()
 int FixTempRescale::modify_param(int narg, char **arg)
 {
   if (strcmp(arg[0],"temp") == 0) {
-    if (narg < 2) error->all(FLERR,"Illegal fix_modify command");
+    if (narg < 2) utils::missing_cmd_args(FLERR, "fix_modify temp", error);
     if (tflag) {
       modify->delete_compute(id_temp);
       tflag = 0;
@@ -205,7 +205,7 @@ int FixTempRescale::modify_param(int narg, char **arg)
 
     int icompute = modify->find_compute(id_temp);
     if (icompute < 0)
-      error->all(FLERR,"Could not find fix_modify temperature ID");
+      error->all(FLERR,"Could not find fix_modify temperature ID {}", arg[1]);
     temperature = modify->compute[icompute];
 
     if (temperature->tempflag == 0)
diff --git a/src/fix_wall_reflect.cpp b/src/fix_wall_reflect.cpp
index ac5940c513..6b5b74c4e0 100644
--- a/src/fix_wall_reflect.cpp
+++ b/src/fix_wall_reflect.cpp
@@ -35,7 +35,7 @@ FixWallReflect::FixWallReflect(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
   nwall(0)
 {
-  if (narg < 4) error->all(FLERR,"Illegal fix wall/reflect command");
+  if (narg < 4) utils::missing_cmd_args(FLERR, "fix wall/reflect", error);
 
   // let child class process all args
 
@@ -53,7 +53,7 @@ FixWallReflect::FixWallReflect(LAMMPS *lmp, int narg, char **arg) :
     if ((strcmp(arg[iarg],"xlo") == 0) || (strcmp(arg[iarg],"xhi") == 0) ||
         (strcmp(arg[iarg],"ylo") == 0) || (strcmp(arg[iarg],"yhi") == 0) ||
         (strcmp(arg[iarg],"zlo") == 0) || (strcmp(arg[iarg],"zhi") == 0)) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal fix wall/reflect command");
+      if (iarg+2 > narg) error->all(FLERR, "Illegal fix wall/reflect {} command: missing argument(s)", arg[iarg]);
 
       int newwall;
       if (strcmp(arg[iarg],"xlo") == 0) newwall = XLO;
@@ -86,26 +86,26 @@ FixWallReflect::FixWallReflect(LAMMPS *lmp, int narg, char **arg) :
       iarg += 2;
 
     } else if (strcmp(arg[iarg],"units") == 0) {
-      if (iarg+2 > narg) error->all(FLERR,"Illegal wall/reflect command");
+      if (iarg+2 > narg) utils::missing_cmd_args(FLERR, "fix wall/reflect units", error);
       if (strcmp(arg[iarg+1],"box") == 0) scaleflag = 0;
       else if (strcmp(arg[iarg+1],"lattice") == 0) scaleflag = 1;
-      else error->all(FLERR,"Illegal fix wall/reflect command");
+      else error->all(FLERR,"Unknown fix wall/reflect units argument: {}", arg[iarg+1]);
       iarg += 2;
 
-    } else error->all(FLERR,"Illegal fix wall/reflect command");
+    } else error->all(FLERR,"Unknown fix wall/reflect keyword: {}", arg[iarg]);
   }
 
   // error check
 
-  if (nwall == 0) error->all(FLERR,"Illegal fix wall command");
+  if (nwall == 0) utils::missing_cmd_args(FLERR, "fix wall/reflect", error);
 
   for (int m = 0; m < nwall; m++) {
     if ((wallwhich[m] == XLO || wallwhich[m] == XHI) && domain->xperiodic)
-      error->all(FLERR,"Cannot use fix wall/reflect in periodic dimension");
+      error->all(FLERR,"Cannot use fix wall/reflect in xperiodic dimension");
     if ((wallwhich[m] == YLO || wallwhich[m] == YHI) && domain->yperiodic)
-      error->all(FLERR,"Cannot use fix wall/reflect in periodic dimension");
+      error->all(FLERR,"Cannot use fix wall/reflect in yperiodic dimension");
     if ((wallwhich[m] == ZLO || wallwhich[m] == ZHI) && domain->zperiodic)
-      error->all(FLERR,"Cannot use fix wall/reflect in periodic dimension");
+      error->all(FLERR,"Cannot use fix wall/reflect in zperiodic dimension");
   }
 
   for (int m = 0; m < nwall; m++)
@@ -170,9 +170,9 @@ void FixWallReflect::init()
     if (wallstyle[m] != VARIABLE) continue;
     varindex[m] = input->variable->find(varstr[m]);
     if (varindex[m] < 0)
-      error->all(FLERR,"Variable name for fix wall/reflect does not exist");
+      error->all(FLERR,"Variable {} for fix wall/reflect does not exist", varstr[m]);
     if (!input->variable->equalstyle(varindex[m]))
-      error->all(FLERR,"Variable for fix wall/reflect is invalid style");
+      error->all(FLERR,"Variable {} for fix wall/reflect is invalid style", varstr[m]);
   }
 
   int nrigid = 0;
diff --git a/src/thermo.cpp b/src/thermo.cpp
index c3cd5adcf5..c4a914e3a3 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -498,14 +498,14 @@ bigint Thermo::lost_check()
 
 void Thermo::modify_params(int narg, char **arg)
 {
-  if (narg == 0) error->all(FLERR, "Illegal thermo_modify command");
+  if (narg == 0) utils::missing_cmd_args(FLERR, "thermo_modify", error);
 
   modified = 1;
 
   int iarg = 0;
   while (iarg < narg) {
     if (strcmp(arg[iarg], "temp") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify temp", error);
       if (index_temp < 0) error->all(FLERR, "Thermo style does not use temp");
       delete[] id_compute[index_temp];
       id_compute[index_temp] = utils::strdup(arg[iarg + 1]);
@@ -540,7 +540,7 @@ void Thermo::modify_params(int narg, char **arg)
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "press") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify press", error);
       if (index_press_scalar < 0 && index_press_vector < 0)
         error->all(FLERR, "Thermo style does not use press");
 
@@ -563,7 +563,7 @@ void Thermo::modify_params(int narg, char **arg)
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "lost") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify lost", error);
       if (strcmp(arg[iarg + 1], "ignore") == 0)
         lostflag = Thermo::IGNORE;
       else if (strcmp(arg[iarg + 1], "warn") == 0)
@@ -571,11 +571,11 @@ void Thermo::modify_params(int narg, char **arg)
       else if (strcmp(arg[iarg + 1], "error") == 0)
         lostflag = Thermo::ERROR;
       else
-        error->all(FLERR, "Illegal thermo_modify command");
+        error->all(FLERR, "Unknown thermo_modify lost argument: {}", arg[iarg + 1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "lost/bond") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify lost/bond", error);
       if (strcmp(arg[iarg + 1], "ignore") == 0)
         lostbond = Thermo::IGNORE;
       else if (strcmp(arg[iarg + 1], "warn") == 0)
@@ -583,11 +583,11 @@ void Thermo::modify_params(int narg, char **arg)
       else if (strcmp(arg[iarg + 1], "error") == 0)
         lostbond = Thermo::ERROR;
       else
-        error->all(FLERR, "Illegal thermo_modify command");
+        error->all(FLERR, "Unknown thermo_modify lost/bond argument: {}", arg[iarg + 1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "warn") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify warn", error);
       if (strcmp(arg[iarg + 1], "ignore") == 0)
         error->set_maxwarn(-1);
       else if (strcmp(arg[iarg + 1], "always") == 0)
@@ -604,18 +604,18 @@ void Thermo::modify_params(int narg, char **arg)
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "norm") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify norm", error);
       normuserflag = 1;
       normuser = utils::logical(FLERR, arg[iarg + 1], false, lmp);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "flush") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify flush", error);
       flushflag = utils::logical(FLERR, arg[iarg + 1], false, lmp);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "line") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify line", error);
       if (strcmp(arg[iarg + 1], "one") == 0)
         lineflag = ONELINE;
       else if (strcmp(arg[iarg + 1], "multi") == 0)
@@ -623,16 +623,16 @@ void Thermo::modify_params(int narg, char **arg)
       else if (strcmp(arg[iarg + 1], "yaml") == 0)
         lineflag = YAMLLINE;
       else
-        error->all(FLERR, "Illegal thermo_modify command");
+        error->all(FLERR, "Unknown thermo_modify line argument: {}", arg[iarg + 1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "colname") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify colname", error);
       if (strcmp(arg[iarg + 1], "default") == 0) {
         for (auto item : keyword_user) item.clear();
         iarg += 2;
       } else {
-        if (iarg + 3 > narg) error->all(FLERR, "Illegal thermo_modify command");
+        if (iarg + 3 > narg) utils::missing_cmd_args(FLERR, "thermo_modify colname", error);
         int icol = -1;
         if (utils::is_integer(arg[iarg + 1])) {
           icol = utils::inumeric(FLERR, arg[iarg + 1], false, lmp);
@@ -646,12 +646,12 @@ void Thermo::modify_params(int narg, char **arg)
           }
         }
         if ((icol < 0) || (icol >= nfield_initial))
-          error->all(FLERR, "Illegal thermo_modify command");
+          error->all(FLERR, "Invalid thermo_modify colname argument: {}", arg[iarg + 1]);
         keyword_user[icol] = arg[iarg + 2];
         iarg += 3;
       }
     } else if (strcmp(arg[iarg], "format") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "thermo_modify format", error);
 
       if (strcmp(arg[iarg + 1], "none") == 0) {
         format_line_user.clear();
@@ -663,7 +663,7 @@ void Thermo::modify_params(int narg, char **arg)
         continue;
       }
 
-      if (iarg + 3 > narg) error->all(FLERR, "Illegal thermo_modify command");
+      if (iarg + 3 > narg) utils::missing_cmd_args(FLERR, "thermo_modify format", error);
 
       if (strcmp(arg[iarg + 1], "line") == 0) {
         format_line_user = arg[iarg + 2];
@@ -692,13 +692,13 @@ void Thermo::modify_params(int narg, char **arg)
           }
         }
         if (icol < 0 || icol >= nfield_initial + 1)
-          error->all(FLERR, "Illegal thermo_modify command");
+          error->all(FLERR, "Invalid thermo_modify format argument: {}", arg[iarg + 1]);
         format_column_user[icol] = arg[iarg + 2];
       }
       iarg += 3;
 
     } else
-      error->all(FLERR, "Illegal thermo_modify command");
+      error->all(FLERR, "Unknown thermo_modify keyword: {}", arg[iarg]);
   }
 }